Auditory distance estimation in song birds: Implications, methodologies and perspectives

Acoustic signals such as bird song degrade progressively during atmospheric propagation and consequently provide information about the distance of the signaler. Information on the signaler's distance is particularly important for animals that use acoustic signals to defend a territory or, in general, to regulate their spacing. Male territorial song birds can use this information to assess the distance of a conspecific singer (called 'ranging'). This ability presumably increases the efficiency of defending a territory because it enables a territory holder to discriminate among threatening intruders and distant conspecifics without interrupting current behavior to, for instance, spend time and energy in approaching. There are a variety of factors that can influence the outcome of 'ranging experiments' of which some are discussed here. So far, playback experiments in the field that impeded close-range experience of subjects with the loudspeaker yielded the clearest evidence for ranging. Flights of subjects to positions beyond the loudspeaker in response to playback of degraded songs provide unambiguous evidence for over-estimation of distance of degraded songs and thus reduce problems of interpretation encountered in experiments that allow subjects' close-range experience with the loudspeaker. Furthermore, the accuracy of ranging can be influenced by the kind of degradation and the availability of song features that facilitate its assessment so that these factors, in addition to an appropriate playback design, should be taken into account in future experiments.